Add external links for tasks with plugin api

app/Core/ExternalLink/ExternalLinkProviderInterface.php

@@ -0,0 +1,71 @@
+<?php
+
+namespace Kanboard\Core\ExternalLink;
+
+/**
+ * External Link Provider Interface
+ *
+ * @package  externalLink
+ * @author   Frederic Guillot
+ */
+interface ExternalLinkProviderInterface
+{
+    /**
+     * Get provider name (label)
+     *
+     * @access public
+     * @return string
+     */
+    public function getName();
+
+    /**
+     * Get link type (will be saved in the database)
+     *
+     * @access public
+     * @return string
+     */
+    public function getType();
+
+    /**
+     * Get a dictionary of supported dependency types by the provider
+     *
+     * Example:
+     *
+     * [
+     *     'related' => t('Related'),
+     *     'child' => t('Child'),
+     *     'parent' => t('Parent'),
+     *     'self' => t('Self'),
+     * ]
+     *
+     * The dictionary key is saved in the database.
+     *
+     * @access public
+     * @return array
+     */
+    public function getDependencies();
+
+    /**
+     * Set text entered by the user
+     *
+     * @access public
+     * @param  string $input
+     */
+    public function setUserTextInput($input);
+
+    /**
+     * Return true if the provider can parse correctly the user input
+     *
+     * @access public
+     * @return boolean
+     */
+    public function match();
+
+    /**
+     * Get the link found with the properties
+     *
+     * @access public
+     * @return ExternalLinkInterface
+     */
+    public function getLink();
+}